1. Field of the Invention
The present invention relates to a modulation circuit for a light emission device, and more particularly to a laser diode modulation circuit of a direct modulation type which applies a modulation signal to a laser diode to produce a intensity-modulated optical signal.
2. Related Background Art
Transmission service of a multi-channel video signal and a CATV system which offers a two-way data communication service have recently been started. The CATV system uses an optical fiber cable to a coaxial cable, and an optical/coaxial hybrid CATV system which is a combination of both systems has also been required. In a VSB-AM (Vestigial Side Band-Amplitude Modulation) optical transmitter in such a system, a laser diode modulation circuit for intensity-modulating the laser diode is used.
A conventional laser diode modulation circuit is shown in FIG. 1. a modulation signal applied to an input terminal 1 is applied to a laser diode (LD) 3 through a variable attenuator 2 to directly modulate it. In order to improve a C/N ratio (carrier to noise ratio), the attenuation by the variable attenuator 2 is selected to a maximum (optimum modulation depth) which does not cause overmodulation of the level of the modulation signal. After the adjustment, the attenuation is fixed for the optical modulation.
However, in the prior art, if the level of the modulation signal varies, the overmodulation or undermodulation occurs. Namely, if the amplitude level of the modulation signal varies due to a temperature change of other factor after the adjustment to the optimum modulation factor, the modulation depth of the laser diode shifts from the optimum. As a result, the transmission characteristic is deteriorated.